1. Field of the Invention
This invention relates to a process for producing polyphenylene oxides of low molecular weight. 2. Description of the Prior Art
Processes for producing polyphenylene oxide by oxidative polymerization of phenols are known. For example, the processes employing copper compounds as catalyst are described in U.S. Pat. Nos. 3,306,874; 3,306,875; and 3,337,499, the processes employing manganese compounds as catalyst in Japanese Pat. Publication Nos. 3195/1967, 30354/1970 and 30355/1970, and British Pat. Nos. 1189405 and 1330326, and the processes employing cobalt compounds as catalyst in Japanese Pat. Publication No. 23555/1970 and German Pat. No. 1720240. These processes are contemplated to produce polyphenylene oxides of high molecular weight having a degree of polymerization of higher than 100, which have been used for various purposes such as molded material utilizing the excellent thermal properties, mechanical property and electrical properties.
Polyphenylene oxides of low molecular weight also have excellent thermal, mechanical and electrical properties, good solubility, film shapeability and adhesivity so that they are expected to be used for various applications such as films, tape, binder, adhesive and plasticizer.
According to the aforementioned prior art processes for producing polyphenylene oxides, polypheylene oxide of high molecular weight is hardly soluble in the reaction medium so that a large amount of a solvent is used in the reaction system to avoid that the resulting polymer becomes insoluble and the propagation of polymerization is inhibited. Usually, there is used a solvent of more than 10 times (by weight) the amount of monomeric phenols and the polymerization reaction is carried out at such low concentration of monomers. In these processes it is also possible to produce polyphenylene oxides of low molecular weight by stopping the reaction of growing the polymers, but the product is so highly soluble in the reaction medium that the yield of polymer is very low and therefore, these are not economically efficient processes for producing polyphenylene oxides of low molecular weight. On the contrary, when the amount of the solvent is decreased and the polymerization reaction is effected at a high concentration of monomeric phenols for the purpose of enhancing the yield of the polymers, such various phenomena are brought about that the period of polymerization is disadvantageously lengthened owing to the lowered polymerization rate, there are produced polymers having branched chain structures due to the presence of monomeric phenols in high concentration in the reaction medium, caused oxidation of side chains of the monomer and polymer and the formation of by-products of diphenoquinone type is inevitable. Thus, the yield is not increased and the resuslting polymer products are remarkably colored and in addition, the distribution of molecular weight in the polymer is very broad. In summary, it is not practically possible to produce preferably polyphenylene oxides of low molecular weight by the aforementioned prior art processes.